Along with recent rapid widespread of the Internet, there is an increasing need to further increase a transmission capacity for a communication traffic volume, increase a transmission speed per system channel, and increase the number of channels based on wavelength division multiplexing (Wavelength Division Multiplexing (WDM)). There is another need to build a flexible network capable of dynamically setting a path in accordance with a traffic volume change or disorders, and to upgrade network infrastructure to provide more variety of services.
To build a high-performance and high-reliability photonic network with such large capacity, a technique of controlling a wavelength is indispensable. A wavelength-variable (selection) light source is a considerably important system key device. In response to the above needs, a wavelength-variable DFB laser has been now put into practical use, which changes a temperature of distributed feedback (Distributed Feedback (DFB)) laser to some extent to realize a wavelength variable range of about 500 GHz (Non-Patent Document 1).
Further, since a wavelength band as wide as 1 THz or more is used (Non-Patent Document 2), or a gain band of an optical fiber amplifier (EDFA: Erbium Doped Fiber Amplifier) doped with rare earth (Erbium etc.) which is used in middle- and long-distance optical communications is about 4 THz, there is a demand to popularize a wavelength-variable laser having a wavelength variable (selection) range of 4 THz or more as a desired wavelength variable range.
As a wavelength-variable laser that can satisfy such demand, extensive studies have been made on an external resonator type wavelength-variable laser that configures an external resonator with a semiconductor light amplifier and an external reflector and inserts a wavelength-tunable (wavelength selection) filter or the like to attain wavelength selection characteristics because a wavelength variable range of 4 THz or more can be attained with comparative ease. Most of the basic characteristics of the wavelength-variable laser of this type are determined based on the wavelength-tunable (wavelength selection) filter inserted into the resonator. Thus, various wavelength-tunable (wavelength selection) filters of excellent characteristics have been developed and introduced into a light source.
For example, there is a filter that rotates an etalon as disclosed in Patent Document 1, a filter that rotates diffraction grating as disclosed in Patent Document 2, and an acousto-optic filter or dielectric filter as disclosed in Patent Document 3. As the wavelength-variable mirror, there is an electrically controlled wavelength-variable mirror as disclosed in Patent Document 4.
There are various methods of configuring an external resonator type wavelength-variable laser by use of the above wavelength-tunable (wavelength selection) filter. In particular, the configuration including “wavelength selection filter+wavelength-variable filter+reflection mirror” in addition to a gain medium as disclosed in Patent Document 5 is effective for realizing a high-performance light source. For example, an etalon having periodic frequency characteristics is used as the wavelength selection filter, and the structure of acousto-optic filter+reflection mirror or an electrically controlled wavelength-variable mirror is used as the wavelength-variable filter+reflection mirror.
The principle of wavelength selecting operations of this configuration is described below in brief with reference to FIG. 16A. First, light emitter from a gain medium such as a semiconductor light amplifier has many Fabry-Perot modes 8 dependent on the total length of an external resonator. Among these modes, some modes corresponding to a cycle of a periodic transmission band 7 of a wavelength selection filter are selected, and the selected modes only pass through the wavelength selection filter.
Next, one mode is selected from the plural modes by use of a wavelength-variable filter as a wavelength-variable filter 7b and the selected mode is transmitted through a filter. The transmitted light is reflected by a reflector or the like and finally returns up to the gain medium to form a feedback loop. With this configuration, a wavelength-variable laser of high mode stability is realized with comparative ease, and wavelength selection characteristics are obtained through relatively simple control.    [Non-Patent Document 1]
“Wavelength Locker integrated DFB-LD module” The 2000 Autumn Meeting of IEICE Communications Society Conference, C-3-106    [Non-Patent Document 2]
T. Morimoto, K. Yashiki, K. Kudo, and T. Sasaki, “Wavelength-selectable microarray light sources for DWDM photonic networks,” (Invited) IEICE Trans. Electron., vol. E85-C, no. 4, pp. 982-989, April. 2002.    [Patent Document 1]
Japanese Unexamined Patent Application Publication No. 4-69987 (FIG. 1)    [Patent Document 2]
Japanese Unexamined Patent Application Publication No. 5-48220 (FIG. 1)    [Patent Document 3]
Japanese Unexamined Patent Application Publication No. 2000-261086 ([0009]-[0010], FIG. 3), ([0011]-[0012], FIG. 5)    [Patent Document 4]
U.S. Pat. No. 6,215,928B1    [Patent Document 5]
Japanese Unexamined Patent Application Publication No. 2002-353555 ([0013])